Possession can be a tricky thing to play in a game. PCs who are possessed will no longer be in control of their actions, and you need to have them behave in certain ways to act out the results of the possession. And those results can often be things detrimental to the success of the party.

Just remember that for PCs to come up with things detrimental to the party, you don't necessarily have to resort to possession.

In the real world of the Egons & Ectoplasms universe, of course, Ghostbusters doesn't exist.
That's the premise of this entire comic, after all. But most of the other things we know and love (such as mad scientists with beautiful daughters)
do, albeit modified by the lack of Ghostbusters:

Without the success of Ghostbusters to spark interest in comedy-horror, Little Shop of Horrors was never revived
with a musical movie and stage show, and remains an obscure film.

Scooby Doo was never made into a movie. Hanna-Barbera fans never had it so good.

The major comedy horror background that pervades all of Western culture is The Munsters, despite it never being much good.
The original film was remade recently into a new, updated television series with a bigger budget, high-tech computerised special effects, and edgy writing.
And it sucked.

Throughout the 1980s, all the greatest Hollywood blockbusters were big-budget romantic thrillers.

Transcript

GM: Suddenly, the phone rings...[SFX]: Riiiing! Riiiing!Spengler: {into phone} Ghostbusters. Dr Spengler speaking.GM: Dana is rambling deliriously. Annie, can you read these lines?Dana: {delirious} I am Zuul... I am The Gatekeeper... Gozer, The Destructor, shall be released...Venkman: {into phone} Venkman. I'm at Dana's place. Looks like she's been possessed. By someone claiming to be The Gatekeeper.Spengler: Gatekeeper?Louis: Gatekeeper! The Keymaster seeks the Gatekeeper!!Spengler: Louis has been possessed by The Keymaster.Venkman: Keymaster? Gate, key... Wait, I get the feeling there's something we need to do here.Venkman: Let's bring them together!Spengler: {to Janine} Venkman's got a plan.Janine: I'll just go increase our business insurance.